In studies done on the
sensitivity of moth optical neurons to light, they have an
extremely low threshold, meaning that even very low levels of
light will allow the moths to "see". Not see
necessarily sharply, but undoubtedly enough to avoid large (dark)
objects.

Why would moths need to
navigate or to orient using light ? All organisms need to
be able to orient themselves in their environment in
relation to other objects (foodplants, mates, finding
shelter, etc.). I grant you that finding foodplants and
mates for moths flying at night is almost exclusively
done using chemical cues. However, when just
"cruising" moths need cues that will allow them
to maintain appropriate orientation to surrounding
objects, the ground, etc., and one of the cues that is
consistent for moths is that light sources (back before
the advent of human-made light sources) were predictably
"UP" (in the sky). I grant you that the moon is
not always at a consistent angle upward in the sky, but
it *is* always up, as are the stars.

Do moths migrate ? The answer is
"yes". Not necessarily in masses like the
Monarch Butterfly, and not necessarily in a particular
direction at a particular time of the year, but moths do
fly long distances and likely use the sky light cues in
the sky *directionally* as well (though little research
has been done on this).

For instance, there is some
documentation for the movement of moths out of the west coast
lowlands of Costa Rica during the dry season up into the moister
mountainous areas to the east, and back down into the coastal dry
forest during the rainy season.

The observation that moths fly more
strongly on humid, cloudy nights is also well documented, but to
say that it doesn't make much sense therefore that they would use
lights as a navigational cue doesn't necessarily follow. Most
animals have *more than one* way to navigate through their
environment, and just because they don't necessarily use one all
the time doesn't mean they *can't*. (Moths *do* have functional
eyes, after all!!).

Just as an off the wall example,
homing pigeons can use sun compass and magnetic cues to migrate
-- a pigeon released on a sunny day with a small magnet attached
to its head (which screws up their magnetic sense) can still
navigate just fine. Unencumbered pigeons do just fine on a cloudy
day as well. However, release a pigeon with a magnet on its head
on a cloudy day and they fly around aimlessly.

Moths undoubtedly can use lights as
a navigational cue, and along with gravitational cues, use the
light sources from above to maintain appropriate
"up-down" orientation in their environment. Noctuid
moths migrate using the moon as a primary reference point. To
calibrate the location in the sky with actual geographical
direction, they periodically use an internal geomagnetic compass.
In fact, every hour, they alter their flight path by 16 degrees
to correct for travel of the moon across the sky (for purists,
rotation of the earth). On moonless nights they navigate solely
with the geomagnetic compass. I guess using the moon is 'easier',
and therefore they 'prefer' that when it is visible, hence the
screwup when bright lights are visible."

Just as an aside, fish are used to
coming to the surface of the ocean during a full moon -- this is
well documented for many fresh water fish as well. Perhaps one of
the reasons is that insects flying through their environment on a
well moonlit night will be confused by *reflected* moonlight from
below and go cascading down into the water (more food for the
fish). I know this has come up in discussions with other
entomologists before, but I am unaware if this is actually
scientifically documented.

Are moths able to hear the light
and come to the light based on certain sound frequencies being
produced by the lights ?

I agree that this may be a
possibility, but there are three things which suggest that this
is certainly not the only, or even an important, mechanism. I
doubt very seriously that white light bulbs and white light bulbs
painted yellow emit significantly different sound impulses, and
so this would not explain why moths come in to different colored
lights in significantly different numbers. Secondly, there are
some families of moths which, in essence, have *no* hearing
capabilities whatsoever (for instance, the Saturniidae [egs.,
Polyphemus, Cecropia, Io, Imperial, etc.]), so without the
ability to hear sound frequencies, saturniids and some other
moths would never end up at lights, and this is certainly not the
case. Lastly, and perhaps most importantly, there would have to
be some reason why the sound is *meaningful* to the moths, and I
doubt that this would be likely for most moth species. Only a few
moths actually use any kind of acoustic communication, so for the
majority of moths, sounds at night are likely to indicate danger
(flee!) as opposed to some sort of attraction.

The use of moon and stars as
navigational cues can at least partly explain why moths end up at
lights. Moths maintain a certain angle between themselves and
light sources, which explains the "spiralling in" that
is easily observable in many species as they come to lights.
Interestingly, some of the species that do this most strikingly,
such as the saturniids, are also some of the species that do not
have hearing apparati. The reason why they stay at the lights
(after "hitting" the moon, an accomplishment they never
evolved a decent response to ) is because it is now bright.

Now close to a bright
light source, the artificial "moon" has become
the "sun", and the moths settle down . . . for
their daytime "sleep". However, I do doubt that
these moths are then as result (at least not the first
time they come into the lights) half-starved. The reason
why some moths simply fall from the wall when tapped has
to do with thermoregulation. Cooled moths (in many cases)
require a certain warm up period before flight
(accomplished by "shivering" [muscular
thermogenesis]). Being half starved is irrelevant for
some species which do not feed as adults, such as the
saturniids mentioned above. Using nighttime celestial
lights sources as navigational cues would also be a
convenient explanation as to why it appears that fewer
moths come to lights on well moonlit nights. Full
moonlight is harder to compete with using the
superstimulating electric light sources, as well as why
it therefore may appear that there is *more activity* at
artificial light sources on cloudy nights, though cloudy
nights also tend to be warmer and more humid, which may
have more of an effect. However, this is certainly not
the entire story. Many, many moths, if you watch them
come to lights, fly *directly* at the light source as
they come in, with little indication of any spiral. Many
moths can hear, and it is these moths which appear to
come more directly at the light. It is possible, and I
stress simply *possible* that these moths are perhaps
using *both* a light and sonic cue to get to the light.
This does *not*, however, explain why the moths
"like" either the light or the sound.

As to the decline of species
because of artificial lighting, the main reason for this
(assuming it does happen) is the predation that occurs on the
placid moths sitting on walls, etc. in the morning (not against
some background that they might be camouflaged on). I have seen
numerous birds "cleaning up" in the early morning, as
well as Bald-Face Hornets and wasps of the genus Tripoxylon.
There may indeed be some selection going on for individual moths
that do *not* respond strongly to light cues (but, of course, if
the navigational argument is sound, it would be impossible to
select this out of a moth species!!).